1. Field of the Invention
The present invention relates to a magnetic recording medium and, more particularly, a coated type magnetic recording medium for broadcasting and computers where high reliability is required.
2. Description of the Related Art
Magnetic recording media have been widely used as sound recording tapes, video tapes, computer tapes, disks, etc. The density of magnetic recording media has been increasing year after year, and the recording wavelength has been shortening. Moreover, a wide range of recording systems including analogue systems to digital systems have been studied.
In particular, a magnetic recording medium in which ferromagnetic metal particles containing Fe as a main component are coated with a binder on a non-magnetic support currently dominates the field of media for broadcasting, video and data recording because of its excellent cost performance. Such magnetic recording media for broadcasting, video and data recording are required to have high electromagnetic conversion characteristics as well as high running durability that can withstand more severe use under a range of conditions than is the case with normal consumer use since recorded images and data are used as commercial products.
In recent years, in order to enhance the recording density the magnetic material used in the magnetic recording medium has been made into finer particles. In addition, since the packing ratio of the magnetic material has improved due to the invention of spindle-shaped fine metal particles having less branching of the particles, it becomes difficult to ensure the running durability due to the occurrence of clogging of the magnetic head (hereinafter, simply called the xe2x80x98headxe2x80x99 in some cases). In recent years in particular, diversity in the application environment has required a higher running durability than is available in the art. In some cases, the maintenance of a VTR is inadequate and it is therefore necessary to increase the allowance for the running durability. Furthermore, the costs for head replacement and head adjustment impose a great burden on the maintenance of VTRs. In comparison with the art, there is thus an increasing demand for a magnetic recording medium that can ensure a long life for the head, that is to say, causes less abrasion of the head.
The running durability includes stills, clogging, decrease in output, etc. In order to improve these properties, the amount of abrasive present on the surface of the magnetic layer has been controlled, as is already known in Japanese Patent Application Laid-open Nos. 61-57036, 64-88917 and 9-35251.
Japanese Patent Application Laid-open No. 61-57036 discloses a magnetic recording medium that improves the abrasivity toward a magnetic head by specifying that the number of abrasive particles per unit area of the surface of a magnetic layer is 0.25 particles/100 xcexcm2. Japanese Patent Application Laid-open No. 64-88917 discloses a magnetic recording medium that improves the abrasivity toward a magnetic head by specifying that the number of abrasive alumina particles per unit area of the surface of a magnetic layer is 0.25 to 0.60 particles/100 xcexcm2. Japanese Patent Application Laid-open No. 9-35251 discloses a magnetic recording medium that causes less head wear by setting the number of abrasive particles exposed by 0.3 xcexcm or more on the surface of a magnetic layer at 3.5 or less particles/100 m2.
Various studies have thus been conducted into the amount of abrasive on the surface, but there is the fundamental problem that adjustment of the abrasivity by using an abrasive cannot simultaneously solve the problems of head wear and head clogging, which are mutually contradictory. That is to say, when the amount of abrasive on the surface is increased so as to enhance the abrasivity toward a magnetic head, although head clogging is suppressed, the head wear increases so shortening the life of the head. When the amount of abrasive on the surface is decreased, although the head wear decreases, the durability is degraded due to head clogging, an increase in dropouts during running, etc. Such problems including head clogging are serious problems in systems for, in particular, broadcasting and computers, and the high cost of head replacement is also a major problem.
As a result of an intensive investigation by the present inventors in order to simultaneously achieve the prevention of head wear and head clogging, which are mutually contradictory, the present invention has been accomplished.
With regard to a magnetic recording medium comprising a non-magnetic support and at least one magnetic layer comprising ferromagnetic metal particles principally containing iron, having a spindle shape and having an average major axis length of 0.15 xcexcm or less and a binder resin, it is an object of the present invention to provide a magnetic recording medium causing less head wear and having excellent running durability in addition to satisfying the required electromagnetic conversion characteristics.
A first object of the present invention relates to a magnetic recording medium comprising a non-magnetic support and at least one magnetic layer comprising ferromagnetic metal particles principally containing iron, having a spindle shape and having an average major axis length of 0.15 xcexcm or less, and a binder resin, and optionally further comprising, between the support and the magnetic layer, at least one adhesive layer or at least one non-magnetic layer principally containing a non-magnetic inorganic powder and a binder, and wherein center line average surface roughness Ra of the magnetic layer surface is 2 to 6 nm, said recoding medium having the following properties:
steel ball wear at 23xc2x0 C. and 70% RH is 2xc3x9710xe2x88x925 to 4xc3x9710xe2x88x925 mm3;
surface C/Fe ratio measured by Auger electron spectroscopy is 50 to 100; and
coefficient of friction between the magnetic layer surface and an SUS rod having a roughness of 8 to 10 nm is 0.2 to 0.3.
A second aspect of the present invention relates to a magnetic recording medium for use in a magnetic recording and playback drive of a middle cylinder rotary type comprising: a non-magnetic support; and at least one magnetic layer formed by coating ferromagnetic metal particles principally containing iron, having a spindle shape and having an average major axis length of 0.15 xcexcm or less, and a binder resin directly on the non-magnetic support or via at least one adhesive layer or at least one non-magnetic layer principally containing a non-magnetic inorganic powder and a binder; center line average surface roughness Ra of the magnetic layer surface being 2 to 6 nm; steel ball wear at 23xc2x0 C. and 70% RH being 2xc3x9710xe2x88x925 to 4xc3x9710xe2x88x925 mm3; surface C/Fe ratio measured by Auger electron spectroscopy being 50 to 100; and coefficient of friction between the magnetic layer surface and an SUS rod having a roughness of 8 to 10 nm being 0.2 to 0.3.